Become Emperor and invade Skyrim? well, you are still emperor but you will also rule over Skyrim; can't really say you're the high King but it's the closest you can get.
I suppose to be HK you'd have to reside in Skyrim as you'd still be a Jarl of one of the holds, and as Emperor, you're 'supposed' to be in Cyrodiil so yeah... you are right in saying it is the closest you can get. At least I think so
